Composite sandwich panels are a special type of composite materials/structures fabricated by attaching two relatively thin but stiff face sheets to a lightweight but thick core. The core material is normally of relatively low strength material, but its higher thickness provides the sandwich composite with high bending stiffness with overall low density. Composite sandwich panels are commonly made by layering a core material between two thin layers that provide strength in tension. The lightweight core is usually attached to the face sheets by adhesive bonding and/or metal brazing. This forms a plate-like assembly.
The face sheets are usually laminates of glass and/or carbon fiber-reinforced thermoplastics and/or thermoset polymers such as unsaturated polyesters, epoxies. Alternatively sheet metal, preferably of lightweight metals such as aluminium, may also be employed for face sheets of sandwich panels. The lightweight core of sandwich panels are usually open- and/or closed-cell-structured foams (such as polyvinylchloride, polyurethane, polyethylene or polystyrene foams, syntactic foams) or open- and/or closed-cell metal foams, preferably of lightweight metals such as aluminium.
Quite often honeycomb structures, preferably of lightweight metals such as aluminium or fibreglass and advanced composite materials, are preferred as lightweight core due to their excellent strength to weight ratio. Honeycomb structures are structures that have the geometry of a honeycomb to allow the minimization of the amount of used material to reach minimal weight. The geometry of honeycomb structures can vary widely but the common feature of all such structures is an array of hollow cells formed between thin vertical walls. The cells are often columnar and hexagonal in shape. A honeycomb shaped structure provides a material with minimal density and relative high out-of-plane compression properties and out-of-plane shear properties.
The behaviour of a composite sandwich panels is orthotropic, hence the panels react differently depending on the orientation of the structure. Therefore it is necessary to distinguish between in-plane forces and out-of-plane forces. In a composite sandwich panel the face sheets are provided for transferring in-plane forces while the lightweight core is provided for high out-of-plane compression resistance.
Composite sandwich panels are widely used where flat or slightly curved surfaces are needed and their high strength-to-weight ratio is valuable. They are widely used in the aerospace industry for this reason, and have been featured in aircraft and rockets for long time. While sandwich panels are capable of carrying high loads, load application and/or anchorage of various components requires specific solutions due to said different strength depending on the direction of the applied forces. In order to make full use of the advanced properties of a composite sandwich panel and to avoid damage thereof, the static and dynamic loads from attached components such as lenses, antennas, etc. must be optimally transferred into the structure. As aircrafts, spacecrafts, rockets, satellites, etc. are subject to strong vibrations on the order of up to 100 g, the loads on the attachment points of components to the sandwich panels carrying them are very high.
To satisfy these strict requirements, the anchorage of components and load application may be achieved by means of inserts fitted into the sandwich structure, such as the insert disclosed in GB 1328429. At the same time, the inserts should be fitted to sit flatly within the composite sandwich panel. This process is currently performed mainly by hand or by complex processes involving labour-intensive production steps requiring various tools and auxiliary means. For example the insert of GB 1328429 is mounted by using an auxiliary plate parallel to the face sheet of the composite sandwich panel, the auxiliary plate being attached to the insert during its insertion into the panel and during the curing of the adhesive which adheres the insert into the panel. However, this can take up to 48 hours which makes this process very inefficient. Furthermore quite often accidental leakage of the adhesive might bond the auxiliary plate to the insert, which might lead to the insert being stressed or in the worst case even pulled out of the sandwich panel during removal of the auxiliary plate, thereby damaging the entire sandwich panel. In addition, known inserts for load application into composite sandwich panels must be very precisely aligned with the cut-outs into which they are mounted, making automated assembly impossible or very costly.